Soft contact lens of the hydrogel type have traditionally been manufactured either by lathe cutting or spin casting. In the lathe cutting method, a lens blank or button of a substantially anhydrous hydrophilic polymer (xerogel) is mechanically cut and polished to a lens shape on a fine lathe, and thereafter contacted with water or saline to hydrate the polymer and form the desired hydrogel lens. The mechanics of the lathe cutting process are similar to those utilized in the manufacture of conventional hard contact lenses, except that allowance must be made for swelling of the lens during hydration of the polymer.
In the spin casting method, a small quantity of hydrophilic monomer is placed in a concave, optically polished mold, and the mold rotated while the monomers are polymerized to obtain a xerogel lens. The two optical surfaces of the lens are formed simultaneously during polymerization, the outer surface being shaped by the concave mold surface, and the inner surface being shaped by the joint actions of centrifugal force generated by the rotating mold and surface tension of the polymerization mixture. The lens produced thereby is contacted with water or saline to hydrate the polymer and form a hydrogel lens as in the case of the lathe cut lens.
The manufacture of soft hydrogel lenses by either lathe cutting or spin casting presents a problem in that when the hydrophilic lens is hydrated, considerable expansion of the lens takes place, and the degree of expansion is not always constant or predictable from lens to lens. In the case of lathe cut lens, variable stresses in the xerogel polymer button may cause differences in the optical properties of the final hydrogel lens. In the case of spin casting, lens properties tend to be more uniform but variability may be introduced by differences in polymerization rate or conditions.
To avoid the difficulties experienced when hydrating a lens initially formed as anhydrous, hydrophilic xerogel, attempts have been made with varying degrees of success to prepare lenses directly in the expanded gel state. Efforts in this direction have generally not been successful in producing high quality hydrogel contact lens and the method has not addopted commercially.
U.S. Pat. No. 3,220,960 (Re. 27,401) suggests direct molding of hydrogel contact lenses by copolymerizing a hydrophilic monomer in aqueous solution with a cross-linking agent to yield an elastic, soft, transparent hydrogel lens. Suitable hydrophilic monomers include esters of acrylic acid and methacrylic acid with alcohols having hydrophilic groups.
U.S. Pat. No. 3,660,545 discloses spin casting soft contact lenses by polymerizing a mixture of a hydrophilic monomer with water or a water miscible solvent. Organic solvents readily soluble in water such as a water soluble lower aliphatic alcohol, or a polyvalent alcohol such as glycol, glycerol, dioxane and the like, are disclosed to be suitable solvents. Solvent content of the polymerization mixture is recommended to be between 5% and 50%, and preferably between 15 and 40% by weight. Solvent content is estahlished to assure that the polymerization mixture will constitute a single phase throughout the polymerization, and that the polymerized lens will still swell appreciably when the solvent is replaced with water.
U.S. Pat. No. 3,699,089 discloses spin casting soft contact lenses by polymerizing hydrophilic monomers in the presence of a water miscible solvent under substantially anhydrous conditions. Suggested solvents are ethylene glycol, glycerol, formamide, dimethylformamide, dimethylsulfoxide, the glycol esters of lactic acid and the liquid polyethylene glycols. The solvent content of the polymerization mixture is recommended to substantially exceed the final equilibrium water level of the hydrogel so that the cast lens shrinks when the solvent is displaced with water.
U.S. Pat. No. 3,780,003 relates to polymers of alkoxy and hydroxy alkyl acrylates or methacrylates and, while not specifically concerned with the manufacture of contact lenses, discloses in Example II the preparation of a transparent, cross-linked gel by polymerizing a mixture of HEMA and EEMA with 40% by volume tetrahydrofuran.
U.S. Pat. No. 4,347,198 discloses a method for static casting or molding contact lenses by polymerizing a mixture of hydrophilic and hydrophobic monomers with from 5% to 95% by weight of a solvent. The solvent is characterized as being one that does not obstruct the polymerization reaction or post cross-linking reaction, and which preferably results in a transparent polymerization product. Where the monomers are N-vinyl pyrrolidone and methyl methacrylate, the solvent is preferably dimethyl sulfoxide and/or ethylene carbonate, optionally with a small amount of dioxane. Other suggested solvents include dimethylformamide, N-methyl pyrrolidone, dimethyl acetamide, and a mixture of ethylene glycol and water.
U.K. Patent Application G.B. No. 2,097,805 discloses a method for static casting or molding soft contact lenses by polymerizing a substantially anhydrous polymerization mixture of acrylic or methacrylic monomers with a water displaceable ester formed of boric acid and a compound containing 3 or more hydroxyl groups. The amount of the boric acid ester diluent in the polymerization mixture is preferably controlled so that the diluent is displaced substantially on a 1:1 basis and lens size and shape is not significantly altered during displacement of the ester with water.
It is apparent from the disclosures of the above patent references that suitable solvents or diluents for specific polymerization svstems have been selected on the basis of trial and error in an effort to identify those materials which will not interfere with the polymerization reaction, can be readily displaced with water following polymerization, and yield a hydrogel contact lens which is optically clear and has good mechanical properties.
We have found that, with the exception of the boric acid ester, the solvents or diluents suggested in the prior art for use in the polymerization of hydrophilic monomers do not yield HEMA based hydrogel contact lenses having the aforesaid desirable optical and mechanical properties. Many of these prior art diluents result in lenses which are cloudy or hazy, near opaque or actually white. Other materials yield lenses which are optically clear but lacking in mechanical properties as indicated by a low modulus of the final hydrogel lens.
The boric acid ester diluent of G.B. No. 2,097,805 can yield a HEMA based hydrogel contact lens with good optical and mechanical properties, but some difficulties may be experienced in molding lenses of relatively thick cross section, i.e., in excess of 1 mm, where displacement of the diluent with water does not occur as rapidly as in the case of a thinner cross section. When the molded lens containing the boric acid ester diluent is placed in water or saline for replacement of the diluent with water, the boric acid ester quickly hydrolyzes to the polyhydric alcohol and boric acid. If the flux of water is high, such as in the case of a very thin contact lens in agitated water, the boric acid will be kept in solution during the replacement process. If on the other hand, the flux of water is low, such as in the case of a lens of thicker cross section and/or in static water, the boric acid may crystallize as sharp needles inside the hydrogel lens with an adverse affect on both mechanical and optical properties.
It is accordingly an object of the present invention to provide a method for selecting suitable diluents for various hydrophilic polymer compositions used in the manufacture of soft hydrogel contact lenses. It is a further object of this invention to define various polymerization systems which include a diluent and are suitable for the preparation of soft hydrogel contact lenses. A yet further object of this invention is to define the parameters of desirable diluents useful in the preparation of soft HEMA based hydrogel contact lenses having desirable optical and mechanical properties. These and other objects of the invention will be apparent from the ensuing description and claims.